Assessing trends in nutrient concentrations in coastal shelf seas: a case study in the Irish Sea

Single point time-series data collected in the Irish Sea since 1954 by Allen et al. (1998) have been re-evaluated using more recent data and areal winter surveys of the Celtic and Irish Seas. Survey data illustrate the heterogeneity of nutrient distributions with N (?28 M) and P (2·0 M) enrichment of the eastern Irish Sea relative to the western Irish Sea (7–8 M N and 0·7 M P). Salinity nutrient relationships demonstrate distinct regional differences throughout the two seas. Trends in the Isle of Man time-series are unlikely to have resulted from changes in analytical procedures and personnel. Concerns over data quality cannot be resolved and there are no independent data to validate the time-series. Including more recent data shows P has declined since the late 1980s and N concentrations have remained stable since the mid 1970s. The absence of a Si trend is consistent with limited anthropogenic influence on riverine Si concentrations. Trends in N and P are consistent with changes in riverine concentrations of these two nutrients and the biogeochemical processes controlling their cycling in shelf waters. Denitrification is the most likely reason for offshore Irish Sea concentrations of N being lower than expected

The Irish Sea: Is it eutrophic?

The question of whether the Irish Sea is eutrophic is addressed by reviewing the evidence for anthropogenic nutrient enrichment, elevated phytoplankton production and biomass and undesirable disturbance in the context of the EU and OSPAR definitions of eutrophication. Winter concentrations of dissolved available inorganic phosphate (DAIP), nitrogen (DAIN as nitrate and nitrite) and silicate (Si) in coastal waters and concentrations of DAIP and Si in offshore waters of the Irish Sea are elevated relative to winter Celtic Sea shelf break concentrations (0.5 μM DAIP, 7.7 μM DAIN and 2.7 μM Si). Significant, negative nutrient salinity relationships and analysis of the Isle of Man nutrient time-series indicate that the elevated Irish Sea levels of DAIP and DAIN are the result of anthropogenic enrichment with highest concentrations (≈2.0 μM DAIP, 30 μM DAIN and 17 μM Si) measured in near shore eastern Irish Sea waters.\ud \ud Summer levels of phytoplankton chlorophyll (Chl) range from <0.1 to 11.4 mg m−3 (mean: 3.4 mg m−3) and from <0.1 to 16.4 mg m−3 (mean: 2.2 mg m−3) in coastal and offshore waters of the western Irish Sea, respectively. Offshore eastern Irish Sea summer chlorophyll levels range from 0.3 to 3.8 mg m−3 (mean: 1.8 mg m−3). Higher levels of spring (up to 43.9 mg m−3) and summer (up to 22.7 mg m−3) biomass in Liverpool Bay are attributed to nutrient enrichment. Estimates of spring and summer production in different regions of the Irish Sea are ≤194 g C m−2.\ud \ud The absence of: (a) oxygen depletion in near shore and open waters of the Irish Sea (except the seasonally isolated western Irish Sea bottom water); (b) trends in the frequency of Phaeocystis spp. blooms and occurrence of toxin producing algae; and (c) changes in the dominant life form of pelagic primary producers, point to a lack of undesirable disturbance and hence argue against anthropogenic eutrophication in the Irish Sea. This conclusion is discussed in the context of future trends in anthropogenic nutrient inputs